Optical transmissions over long distances (several hundred to several thousands of km) use monomode optical fibers. These offer the advantage of not exhibiting modal dispersion (apart from polarization modal dispersion) and of being able to withstand high outputs of several tens of Gbits/s per wavelength, and to do so for a plurality of wavelengths.
Nevertheless, for transmissions over short distances, particularly for wide band local area networks (LANs), multimode or multi-core fibers constitute a particularly interesting alternative because they make it possible to use low cost components (plastic or POF fibers).
Silica multi-mode fibers have a core of large diameter allowing the propagation of several guided modes, noted Llp for a rectilinear polarization where l is the azimuthal mode index and p the radial mode index. The mode LP01 is the fundamental mode, the only one able to propagate in a monomode fiber. The total number of modes Llp depends on the optogeometric parameters (diameter of the core, particularly the index profile). The information to be transmitted is spread out on the different guided modes. The pass band of the multimode fibers is then limited by intermodal dispersion. Normal multimode fibers (core of diameter 62.5 μm instead of 8 to 10 μm for monomode fibers) enable the propagation of several hundreds of modes.
When the number of modes Llp is low (typically from 2 to 10 modes, corresponding to values of the normalized frequency parameter V<8)), one speaks of weakly multimode fibers or instead FMF (Few-Mode Fibers).
In their applications to optical telecommunications, FMF are exploited essentially for propagation according to the fundamental mode.
FMF fibers currently provide a good compromise between monomode fibers and standard multi-mode fibers (several hundreds of modes) in that they make it possible to attain a high pass band x length of fiber product.
The pass band of multimode fibers is generally greater than that of monomode fibers, each mode being separately modulated and the signal to be transmitted being multiplexed on the different modes. This pass band is nevertheless limited by the coupling between modes Llp during propagation (inter-mode crosstalk). Furthermore, on account of the imperfections and non-homogeneities of the fiber, the different modes do not undergo the same attenuation. The loss differential between the modes Llp, also designated MDL (Mode Dispersion Loss), induces an increased sensitivity to noise sources, which can significantly limit the range of these systems.
Multi-core fibers comprise a plurality of cores (generally from 2 to 7 cores) within a common sheath. The dimension of the cores is sufficiently small to only enable a monomode propagation in each of them. Unlike multimode fibers, these thus do no exhibit modal dispersion. On the other hand, evanescent waves create a coupling between the different cores (inter-core crosstalk), the level of crosstalk is all the higher when the number of cores is high and the inter-core distance is low. Like the inter-mode coupling evoked previously, inter-core coupling limits the range of these systems.
Whatever the type of fiber, another limitation of the pass band is due to Polarization Dependent Loss or PDL and to Polarization Mode Dispersion or PMD. In fact, in an ideal fiber, two signals polarized rectilinearly according to two orthogonal axes undergo the same attenuation and propagate at the same speed. However, in practice, asymmetry defects and random imperfections of the fiber affect differently two orthogonal polarizations and lead to a degradation of the signal, which limits the maximum output that can be attained on the fiber.
An additional limitation appears when the luminous power injected into the fiber is sufficiently high to generate therein non-linear effects. This will in particular be the case when one has to resort to optical signals of high intensity to compensate the attenuation of the fiber for transmission over a long distance.
This limitation appears in particular when a wavelength multiplexed transmission or WDM (Wavelength Division Multiplexing) is used.
In fact, a high intensity wave transmitted at a first wavelength can modify by Kerr effect the index of the fiber at a second wavelength close to the first. More generally, when two waves propagate in an optical fiber, one observes a phase modulation of one as a function of the intensity of the other and vice versa. This phenomenon, known as Cross Phase Modulation or XPM, is all the more sensitive when the luminous intensities in play are important and when the wavelengths are close. It thus affects in the first instance WDM systems with high spectral density, also known as DWDM (Dense WDM), operating over a long transmission distance (long haul). This phenomenon is particularly marked when both optical signals at low output, intensity modulated, by OOK (On Off Keying) modulation, and optical signals at high output, phase (PSK) and/or amplitude (QAM) modulated propagate in the fiber. The signal to noise ratio upon reception of these phase and/or amplitude modulated signals will be all the more degraded when their spectral efficiency, or in an equivalent manner, their order of modulation, is higher.
The basic problem of the invention is to become free of the limitations respectively due to intermodal crosstalk and to inter-core crosstalk.
A first subsidiary problem is moreover to become free of the phenomenon of polarization dependent loss (PDL)/polarization modal dispersion (PMD) when the system for transmission uses polarization multiplexing.
A second subsidiary problem is moreover to become free of crossed phase modulation (XPM) when the system for transmission uses wavelength multiplexing.